Setting Deck Posts From the Top Down
Sometimes it's better to install the beams first, then cut the posts to length.
When a beam spans between two points of the deck that are already installed, it is easier to put the beam in place and cut the posts to length rather than trying to use a control line to determine the post length.
Once you’ve set the beam in place, run a string along one edge (see images below), and check the beam for straightness. If the beam has a bow or sag to it, correct it before measuring for post height. If you don’t, you’ll build this flaw into the deck. You may have to prop the beam up or weigh it down accordingly.
1. Check the beam for straightness by stretching a string along one edge and sighting along the beam and string.
Trade Secret
If the beam has a sag to it, you might be tempted to prop it up with a 2×4 cut to length, but there’s a better way. To prop up a beam without cutting lumber, simply drive a nail into the side and use that to lift the beam. The nail won’t hold a lot of weight but it will hold enough to lift the beam 1/4 in., which can be enough to allow you to slip the post into place without having to fight with it.
To install a post under the beam, you’ll first need to position the post-base hardware and then mark the post length. To position the hardware, place the post on the footing with the top against the beam plumb in both directions. When plumb, the post is adjacent to where it will be installed. Use the edges to mark the final location on the underside of the beam. Before moving the post, mark the correct location on the footing as well. Use the post to mark one side and a scrap of wood aligned with the side of the post to mark a second side of the final post location. In theory, you could use the marks to find the center of the post base by simply aligning the base to the marks. However, this is slightly inaccurate and a more precise method is to use a plumb bob. Finally, place the post-base hardware on the footing. The edges and bolt hole should all align with the marks you just made. Now install the hardware per the manufacturer’s instructions.
Once the post base is installed, determine the post length by placing the post’s edge on the base and holding it to the marks you made on the beam’s underside. Cut the post to length and treat the bottom end with wood preservative and cap the top with self-adhesive flashing. In an ideal world the post will slip into place with a slight upward lift to the beam.
2. Plumb a post (not cut to length) adjacent to the final install location.
3. Mark the post center and edges on the underside of the beam.
4. Mark two sides of the post location on the base using one side of the post and a scrap of wood set at a right angle.
5. Mark the post center on the footing with a plumb bob aligned with the post’s centerline marked on the beam.
6. Use the marks to align the post base on the concrete footing.
7. Mark the post to length by positioning it as plumb as possible in the post base.
8. Slip the post into place and temporarily secure it with a screw until the final hardware is installed.
What Can Go Wrong
Don’t trust the factory to make perfect cuts. Sometimes the factory cuts look as if they were made by a blind chimp wielding a chainsaw. If the cut is off, take the time to square it up and give it a coat of wood preservative.
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